Subscriptions Losing Sync

For some reason, some students signing their plan are being deleted from their accounts, and since I can not manually set the time, only those already pre-ready (1 month, 3 months, 6 months and 1 year), it ends up some students who have already had 3 months of access, I have to put back 1 year again, in the case of the annual purchase.
What could be happening? This has been happening for some time, and more and more often.

Ihor, I do not use any signature extensions, just the theme itself.
Over time, I know it is not possible, just because many students are simply being excluded from the plans, even before the deadline ends. And this has been happening in all kinds, being 1 year, 6 months, 3 months, 1 month … What could be causing this?